Ultrasonic flowmeter and method of metering

ABSTRACT

In an ultrasonic flowmeter first ultrasonic signals are caused to traverse in both directions the fluid carrying conduit at an angle with respect to the direction of fluid flow. The times t1 and t2 necessary for the signals to traverse in the one and in the other direction are measured. By means of a second ultrasonic signals traversing said conduit normal to the direction of fluid flow in a time to to a train of signal pulses with a periodicity of t0 is generated for obtaining with the aid of three counters, (t2 - t1(/t02 which is a measure of the flow rate and from which the uncertain term representing the speed of the ultrasonic signal in the fluid is eliminated.

United States Patent 1 Courty 3,731,532 May 8, 1973 [54] ULTRASONIC FLOWMETER AND METHOD OF METERING [75] Inventor: Albert Courty, Paris, France V [73] Assignee: Thomson-CSF, Paris, France [22] Filed: Jan. 14, 1972 [21] Appl. No.: 217,753

52 US. Cl. ..73/194 A [51] Int. Cl ..G01p 5/00, G0lf 1/00 [58] Field of Search ..73/l94 A [56] References Cited UNITED STATES PATENTS 3,336,801 8/1967 Snavely ..73/194 A X 3,402,606 9/1968 Bruha ..73/194 A 3,537,309 11/1970 Geohegan, Jr. et a]... 73/194 A 3,631,719 1/1971 Charvier ..73/l94 A P rim ary Examiner Charles A, niieiii" Attorney-Edwin E. Greigg 57 ABSTRACT In an ultrasonic flowmeter first ultrasonic signals are caused to traverse in both directions the fluid carrying conduit at an angle with respect to the direction of fluid flow. The times t and t, necessary for the signals to traverse in the one and in the other direction are measured. By means of a second ultrasonic signals traversing said conduit normal to the direction of fluid flow in a time t, to a train of signal pulses with a periodicity of t is generated for obtaining with the aid of three counters, (t, hut, which is a measure of the flow rate and from which the uncertain term representing the speed of the ultrasonic signal in the fluid is eliminated.

3 Claims, 5 Drawing Figures AMPLIFIER UP8 DOWN BSTABLE UP AND DOWN coumER STAGE COUFITER BlsTABLE CLOCK STAGE coimThR PROGRAMMLR r min a ULTRASONIC F LOWMETER AND METHOD OF METERING BACKGROUND OF THE INVENTION This invention relates to an apparatus for measuring the flow ofa fluid, such as a liquid, in a conduit.

Flowmeters are known lwhich utilize the measured time period necessary foran ultrasonic wave to travel from one wall portion of the conduit to another.

The aforenoted duration is a function of the speed of displacement of the fluid and the traveling speed of sound in the fluid. It is a disadvantage of flowmeters of this type that the last-named speed is not always known with high precision. 1

Furthermore, if the liquid is non-homogeneous as it may be the case in oil conduits where different types of oil derivatives may flow in the conduit in succession, the traveling speed of sound may vary between relatively wide limits.

French Patent No. 1,375,940 discloses a flowmeter which, by means of two successive measurements and with the aid of a numeric calculator, permits an elimination of the influence of the traveling speed of sound in the fluid.

The disadvantage of the aforenamed flowmeter resides in the fact that it requires the use of a numeric calculator, which substantially increases the costs involved.

OBJECT AND SUMMARY OF THE INVENTION further objects and advantages will become more aptransducer 12 by means of a switch 3 actuated by a programmer 4. The electric outputs of the two transducers 11 and 12 are connected to the inputs of a forward and reverse counter 5 for controlling the operation of the latter. The counter 5 counts the pulses of a clock pulse generator 6 activated by the programmer 4. The latter also sets the counter 5 into a state for reverse counting. The pulses which the counter 5 emits during its reverse counting maintain a bistable stage 7 open until the counter reaches zero.

Two additional transducers 21 (receiver) and 22 (emitter) are placed at diametrically opposite locations of the conduit 1, for example, downstream of the transducers 11 and 12. The output of the transducer 21 is connected to the input of the transducer 22 through an amplifier 8. Thus, when the transducer 22 emits an ultrasonic pulse, the latter is received by the transducer 21 which, in response, emits an electric pulse applied to the input of the transducer 22 through the amplifier 8. In this manner a series of pulses are generated, the periodicity of which will be equal to the duration t required for the ultrasonic wave to travel from the transducer 22 to the transducer 21 The transducer 22 is activated by the programmer 4.

The pulses received by the: transducer 21 are counted successively by two counters 9 and 10. The counter 9 is activated by the bistable stage 7, while the counter 10 is activated by an additional bistable stage a 13. The latter, in turn, is controlled by the counter 9 in parent, from the ensuing detailed specification of a I preferred, although exemplary, embodiment taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING SUMMARIZED DESCRIPTION OF THE MEASURING SYSTEM AND ITS COMPONENTS Turning now to FIG. 1, there is shown a fluid carrying conduit 1 in which the fluid flows with a speed V in the direction of the axial arrow. 7

Two first electroacoustic transducers 11 and 12 are disposed on two generatrices of the conduit I belonging to the same diametral plane. The transducer 11 is disposed upstream of the transducer 12. Each transducer ll, 12 is adapted to emit an ultrasonic beam and the same manner as the bistable stage 7 is controlled by the counter 5. The counter 9 is also controlled by the programmer 4. The output of the counter II) is con-.

I OPERATION OF THE SYSTEM SHOWN IN FIG. 1

The signal generator 2 which, in a first measuring period is connected to the transducer 11 by the switch 3, emits a train of waves having a frequency chosen in such a manner that the difference in phase due to the traveling time of the wave from the transducer 11 to transducer 12 or conversely, is less than 211'.

Subsequent to the termination of the transitory period and from the beginning ofa moment determined by the programmer 4, the first passage through zero of the emitted electric signal with a positive slope is marked at an instant t (FIG. 2).

The counter 5 and the clock 6 are activated at the moment t The counter 5 counts the clock pulses of the clock 6 until, at the moment t',, it is stopped by the transducer 12. This moment t, is the first instant succeeding t when the electric signal generated by the transducer 12 in response to the ultrasonic signal emitted by the transducer 11 traverses the zero level with a positive slope. Thus, time r is the period necessary for the ultrasonic signal to travel from transducer 11 to transducer 12.

At that moment the measuring process is stopped. The counter 5 has counted N, clock pulses.

In a second measuring period the switch 3 connects the transducer 12 with the generator 2. The measurement is effected in the same manner as precedingly described, except that now the counter 5 is set into a position for reverse counting until it reaches zero (erasure of its contents), whereupon it is set into a forward counting position. The counter 5 then counts forward until the moment t at which time the electric signal transmitted by the transducer 11, in response to the receipt of the ultrasonic signal emitted by the transducer 12, passes through the zero level with a positive slope and arrests the counter 5. The latter then has a count of N which is equal to t t, with an accuracy within one clock pulse period.

Thus,

t, L/(c+ v cos),

t L/(c v cos0) where L is the distance from the transducer 11 tothe transducer 12,

c is the speed of the ultrasonic signal in the fluid,

v is the speed of the fluid and co s 0 is the angle between the two directions 11-12 and V.

The expression t t, is, since v is negligible with respect to approximately equal to Knowing N v can be determined if L and c are known. Thus, as it has been mentioned before, c is not always constant and furthermore its value is not always known with precision. In order to eliminate this term, the flow metering process includes two further measuring periods.

In the third measuring period the counter 5 is placed into the reverse counting position by the programmer 4. The clock pulses now counted by the counter 5 may have a frequency which is much smaller than during the first two counting periods. In fact, in the first two counting periods, the clock pulse frequency had to be large with respect to that of the ultrasonic wave in order to obtain a good resolution. As it will be seen, the inverse of the above is now of interest.

In the third measuring period the transducer 22 is caused to operate. It transmits an ultrasonic signal which is received at the end of time 1 by the transducer 21. The latter, in response, transmits an electric pulse amplified by the amplifier 8 and applied to the input of the transducer 22. A coupling is thus constituted and a series of pulses is emitted with the periodicity of In order to eliminate the factor 0, it is sufficient to measure t and then form the quotient in which there appear only v and constant terms.

It may be demonstrated that 1 is equal to D /c wherein D is the diameter of the fluid conduit 1.

The aforedescribed operation is effected by means of counters 9 and 10.

The counter 9 counts the pulses of t periodicity as long as it is maintained active by the bistable stage 7,

that is, as long as the counter 5 is not empty. Upon such occurrence, the counter 9 is stopped.

The number N contained in the counter 9 is then proportionate to t,)/t,,. In fact, the period necessary for the counter 5 to empty is proportionate to t, t, and the number which is indicated by the counter 9 is inversely proportionate to t that is, proportionate to f l/t the frequency of the recycling pulses.

The same operation, in a fourth measuring period, is effected by means of the counter 10, while the counter 9 is placed in the position of reverse counting by the action of the same clock pulse generator.

In this manner the sought result is obtained: the counter 10 indicates a number N, proportionate to (t z,)/t,, A signal representing the number N, is applied by the counter 10 to the display device 14.

DESCRIPTION OF THE CIRCUIT USED IN THE FIRST TWO MEASURING PERIODS FIG. 3 shows a diagram of an exemplary circuit for effecting measurements in the first two measuring periods.

The circuit shown in FIG. 3 comprises a first flip-flop circuit FF, which receives from the programmer (not shown in FIG. 3) an authorization to measure. It is thus set into the ONE state and delivers a voltage of level 1 which is applied to the gates AND, and AND The two gates AND, and AND are connected to the two inputs of a flip-flop circuit FF The latter is set to the ONE state when the wave from the transduceremitter traverses for the first time the zero value. The signal corresponding to this occurrence is transmitted through the second input of the circuit AND,. The flipflop circuit FF remains in its ONE state until it is reset to the ZERO state which is impressed upon it by the circuit AND, at the moment when the signal of the transducer-receiver first passes the zero value. The same process takes place in the two directions of measurement.

When the flip-flop circuit FF is in its ONE state, it opens a gate circuit AND The latter receives through its second input the clock pulses having a frequency in the order of 1 MHZ, for example. The output of the gate AND, is connected through a circuit OR, to the input of the counter C,. The latter has a function analogous to that of counter 5 illustrated in FIG. 1.

The first group of components permits the performance of measurement in the first period, that is, the measurement of period t, (the upstream transducer 11 then being the transmitter). Thus, the flip-flop circuit FF set to the ONE state by the programmer, delivers a voltage which opens the gates AND, and AND The wave emitted by the electric signal generator 2 (FIG. 1) is transformed first by incrementation and then by derivation into a series of positive pulses (FIG. 4) with the elimination of the negative pulses. Such series of positive pulses are generated each time the alternative primary wave passes zero with a positive slope. At the first pulse which follows the authorization for measurement, the flip-flop circuit FF, is set into the ONE state by the gate AND,. At the appearance of the first succeeding pulse on the receiver, the flip-flop circuit FF, is reset to the ZERO state. The gate AND, is thus open during the time t,.

The counter C, thus counts the clock pulses from the clock 6 received through a circuit AND, and the gate OR,. The counting is stopped when the flip-flop FF, is closed. Thus, the duration of counting is t,. The counter 5 indicates a number N, which, with an accuracy within one clock pulse period, is equal to t,.

The components hereinafter described permit the performance of a measurement in the period t, 1,.

The counter C, is set into its reverse counting position by the flip-flop circuit FF, upon a command signal from the programmer through the differentiator D The stages of the counter C, are connected to a circuit AND which is connected to the circuit AND, to open the latter when the counter C, reaches zero, thus delivering a second authorization to measure to the gate AND The aforenoted second authorization to measure is necessary for stopping the counter C, at the end of a period which, in reality, corresponds to the phase difference to be measured irrespective of the relative position of the transmitted and received waves.

The operation of the above described circuit group will be better understood with reference to. the time diagram shown in FIG. 4. i

The programmer (not shown) effects the following sequential operations:

a. At the moment t, there begins a measurement with which A t, is determined. The flip-flop FF, causes at a first input the setting of the flip-flop FF, into its ONE state and thus measuring is effected as it has been seen hereinbefore while the counter C, is counting.

b. At the moment t, which indicates the end of the first sequence, a negative pulse is generated by the differentiator D,. The latter causes the flip-flop FF, to be reset into its ZERO state and sets the counter C, into a reverse counting position at D. I

At the moment t the flip-flop FF is reset to ZERO state; it thus closes the gate AND- The reverse counting starts in the manner of the preceding forward counting. The counter, however, can be set to its forward counting position only after it has reached its zero value. At that moment a pulse is generated by the circuit AND, which sets the flip-flop FF, to its ONE state. Thereupon the gate AND, opens, the measuring starts in the sense of forward counting and stops when the receiver receives a wave passing through zero in the increasing sense (positive pulse).

At that moment the counter will have registered a count A t, A t,. It is known that A 1 is necessarily greater than A t, and that the first phase of reverse counting has erased the entire contents of counter C,, that is, the equivalent ofA t,.

All pulse occurrence corresponding to A t, At, has to be discarded and must have no effect on the measurement.

When the measuring is terminated, the circuit AND, and the differentiator D, generate a negative pulse which resets the flip-flop FF,

The counter C, is reset'to a reverse counting position.

DESCRIPTION OF THE CIRCUIT USED IN THE THIRD AND FOURTH MEASURING PERIODS FIG. 5 shows the circuit group which is adapted to effect measurements of 6 0/ 0 and 2 l)/ 0 In FIG. 5 there are shown components also illustrated in FIG. 3, such as the flip-flop circuit F F, actuated at one of its inputs by the programmer.

The flip-flop circuit FF, in its ONE state applies its output voltage to a circuit AND The latter, being open, delivers to the counter C, clock pulses of a frequency much smaller (in the order of 10 KHZ) than those referred to earlier.

The counter C, which is in the position of reverse counting thus receives through the circuit AND, clock pulses of relatively low frequency. The flip-flop circuit FF.,, from the moment of the coupling of the transducers 21 and 22 and as commanded by the programmer, is set to its ONE state. It then authorizes the emptying of counter C, at the rate of 10 KHz and the forward counting by a counter C corresponding to counter 9 in FIG. 1. The flip-flop FF, is reset to the ZERO state by the counter C, when the latter reaches zero. Then the pulse delivered by C, to the second input of the flipflop F F, resets it to its ZERO state.

On the other hand, the flip-flop circuit FF opens a circuit AND,, the output of which is connected to the counter C, for making the measurement (t, t,)/t The circuit AND, receives, on the other hand, recurring coupled pulses of the transducers 21 and 22.

The above-described circuit group operates as follows:

The first recurring pulse sets the flip-flop FF, into the ONE state and the flip-flop FF into the ZERO state.

The gate AND, is open. The counter C, starts its reverse counting of the clock pulses supplied at a rate of 10 KHz by the clock. Since the gate AND, is opened by the flip-flop F F the counter C,, counts the recurring pulses delivered by the coupled transducers.

When the counter C, again reaches zero, the flip-flop FF, is reset to its ZERO state and the operations are stopped. As the gate AND, closes, the counter C, has counted pulses at the frequency f,, l/t during a period necessary for the counter C, to be emptied, that is, during a period proportionate to t, t,. It will thus indicate a counterproportionate to (t, t,)/t,,.

The other components of the circuit permit the performance of the measurement (t )/t,,

The flip-flop circuit FF, is connected by means of a differentiator D to one input of a. flip-flop circuit FF The other input of the latter is connected to the output of the counter C, which delivers thereto a pulse when it reaches zero.

The flip-flop circuit FF, is connected to the input D of the counter C and commands its reverse counting. It is also connected to a gate AND, which, when open, receives low frequency clock pulses. The recurring pulses are forwarded to a counter C,, (which corresponds to the counter 10 in FIG. 1) through a gate AND, which is open when the flip-flop circuit FF, is in its ONE state. The output of the counter C is connected to a display device A.

The aforedescribed assembly operates in the following manner:

The flip-flop circuit FF.,, when it is in its ZERO state, causes the difi'erentiator D to generate a negative pulse which sets the flip-flop circuit FF, into its ONE state. In this latter position, the flip-flop circuit FF,,, on the one hand, opens the gate AND, which permits the feeding of the counter C in its reverse counting position. Then the counter C is emptied at the rate of the clock pulses. When it reaches zero, it resets the flipflop circuit FF into its ZERO state.

The bistable circuit FF on the other hand, also opens in its ONE state the gate AND which permits the feeding of the counter C Thus, the latter then counts the recurring pulses produced by the transducers; this occurs solely during the period when the flipflop circuit FF is in its ONE state.

The count obtained at the end of the counting process is (t t )/t for the same reasoning as applied to the counter C The display device A then indicates the speed v of the fluid in the conduit 1.

That which is claimed is:

1. In an ultrasonic flowmeter for measuring the speed of a fluid flowing in a conduit, said flowmeter being of the known type that has (a) two first electroacoustic transducers disposed on the wall of said conduit at opposite sides of the same diametral plane and spaced in the direction of fluid flow, both said first transducers emitting ultrasonic signals and receiving ultrasonic signals emitted by the other after they traverse said conduit at a predetermined inclined angle with respect to the direction of fluid flow, each first transducer having an electric output on which an electric signal appears upon receipt of an ultrasonic signal from the other first transducer, (b) means for alternately and oppositely switching said first transducers to reception and transmission, means for measuring t t,, wherein is the traveling time of the ultrasonic signals from the downstream transducer to the upstream transducer and I is the traveling time of the ultrasonic signals from the upstream transducer to the downstream transducer, (d) two second electroacoustic transducers disposed on the wall of said conduit at diametrically opposed locations, one of said second transducers emitting ultrasonic signals and the other of said second transducers receiving the same, (e) means for coupling said second transducers to one another to cause the appearance at an output of the receiving second transducer a train of electric pulses with a periodicity of wherein t is close to t and t and is the traveling time of the ultrasonic signals from one of said second transducers to the other, (f) means for measuring and (g) means for determining the term (t t,)/t representing the speed of said fluid in said conduit, the improvement comprising,

A. a clock transmitting electric clock pulses,

B. a first counter connected to said clock and to said output of each first transducer,

C. means connected to said first counter for causing the latter to start counting forward with the rate of said clock pulses at the moment said upstream transducer emits an ultrasonic signal,

D. means connected to said first counter to stop its forward counting upon receipt of said last-named ultrasonic signal by said downstream transducer,

E. means connected to said first counter for causing the latter to start counting in reverse at the moment said downstream transducer emits an ultrasonic signal,

F. means connected to said first counter to cause the latter to start counting forward at the moment it reaches zero durin its reverse counting, G. means connecte to said first counter to stop its forward counting upon receipt of the last-named ultrasonic signal by said upstream transducer to contain a count of H. a second counter,

1. means operatively connecting said second counter with said first counter and with said output of said receiving second transducer to count pulses of t periodicity during a reverse counting of said first counter as long as the latter has contents other than zero, at which moment said second counter contains a count proportionate to (t )/r J. a third counter,

K. means operatively connecting said third counter with said second counter and with said output of said receiving second transducer to count pulses of t periodicity during a reverse counting of said second counter as long as the latter has contents other than zero, at which moment said third counter contains a count proportionate to (t 1 V102- 2. A method of measuring the speed of a fluid in a conduit, comprising the following steps:

A. transmitting an ultrasonic signal acrosssaid con duit in the downstream direction at an inclined angle to the direction of fluid flow,

B. counting clock pulses during the traveling time t, of the ultrasonic signal transmitted according to step (A) to determine t,, said steps (A) and (B) constituting a first measuring period,

C. transmitting an ultrasonic signal across said conduit in the upstream direction at said angle,

D. counting clock pulses during the traveling time t of the ultrasonic signal transmitted according to step (C) to determine and the difference t t said steps (C) and (D) constituting a second measuring period,

E. transmitting ultrasonic signals diametrically across said conduit with a periodicity of t which is the traveling time of the last-named ultrasonic signals diametrically across said conduit,

F. generating a train of electric pulses with a periodicity of t in response to the ultrasonic signals transmitted according to step (E),

G. counting the electric pulses generated according to step (F) for a period proportionate to t t, to determine a term proportionate to t,)/t said step (G) constituting a third measuring period and H. counting the electric pulses generated according to step (F) for a period proportionate to (t 1, )/t to determine the term (t t,)/t representing the speed of the fluid in said conduit, said step (H) constituting a fourth measuring period.

3. A method as defined in claim 2, wherein the counting steps (B) and (D) are performed by a first counter at a relatively high clock pulse frequency, the counting step (G) is performed by a second counter during an emptying of the contents t of the first counter at the rate of a relatively low clock pulse frequency and the counting step (H) is performed by a third counter during an emptying of the contents (t n )/t of the second counter at the rate of said relatively low clock pulse frequency. 

1. In an ultrasonic flowmeter for measuring the speed of a fluid flowing in a conduit, said flowmeter being of the known type that has (a) two first electroacoustic transducers disposed on the wall of said conduit at opposite sides of the same diametral plane and spaced in the direction of fluid flow, both said first transducers emitting ultrasonic signals and receiving ultrasonic signals emitted by the other after they traverse said conduit at a predetermined inclined angle with respect to the direction of fluid flow, each first transducer having an electric output on which an electric signal appears upon receipt of an ultrasonic signal from the other first transducer, (b) means for alternately and oppositely switching said first transducers to reception and transmission, (c) means for measuring t2 - t1, wherein t2 is the traveling time of the ultrasonic signals from the downstream transducer to the upstream transducer and t1 is the traveling time of the ultrasonic signals from the upstream transducer to the downstream transducer, (d) two second electroacoustic transducers disposed on the wall of said conduit at diametrically opposed locations, one of said second transducers emitting ultrasonic signals and the other of said second transducers receiving tHe same, (e) means for coupling said second transducers to one another to cause the appearance at an output of the receiving second transducer a train of electric pulses with a periodicity of t0, wherein t0 is close to t1 and t2 and is the traveling time of the ultrasonic signals from one of said second transducers to the other, (f) means for measuring t0 and (g) means for determining the term (t2 - t1)/t02 representing the speed of said fluid in said conduit, the improvement comprising, A. a clock transmitting electric clock pulses, B. a first counter connected to said clock and to said output of each first transducer, C. means connected to said first counter for causing the latter to start counting forward with the rate of said clock pulses at the moment said upstream transducer emits an ultrasonic signal, D. means connected to said first counter to stop its forward counting upon receipt of said last-named ultrasonic signal by said downstream transducer, E. means connected to said first counter for causing the latter to start counting in reverse at the moment said downstream transducer emits an ultrasonic signal, F. means connected to said first counter to cause the latter to start counting forward at the moment it reaches zero during its reverse counting, G. means connected to said first counter to stop its forward counting upon receipt of the last-named ultrasonic signal by said upstream transducer to contain a count of t2 - t1, H. a second counter, I. means operatively connecting said second counter with said first counter and with said output of said receiving second transducer to count pulses of t0 periodicity during a reverse counting of said first counter as long as the latter has contents other than zero, at which moment said second counter contains a count proportionate to (t2 - t1)/t0, J. a third counter, K. means operatively connecting said third counter with said second counter and with said output of said receiving second transducer to count pulses of t0 periodicity during a reverse counting of said second counter as long as the latter has contents other than zero, at which moment said third counter contains a count proportionate to (t2 - t1)/t02.
 2. A method of measuring the speed of a fluid in a conduit, comprising the following steps: A. transmitting an ultrasonic signal across said conduit in the downstream direction at an inclined angle to the direction of fluid flow, B. counting clock pulses during the traveling time t1 of the ultrasonic signal transmitted according to step (A) to determine t1, said steps (A) and (B) constituting a first measuring period, C. transmitting an ultrasonic signal across said conduit in the upstream direction at said angle, D. counting clock pulses during the traveling time t2 of the ultrasonic signal transmitted according to step (C) to determine t2 and the difference t2 - t1, said steps (C) and (D) constituting a second measuring period, E. transmitting ultrasonic signals diametrically across said conduit with a periodicity of t0 which is the traveling time of the last-named ultrasonic signals diametrically across said conduit, F. generating a train of electric pulses with a periodicity of t0 in response to the ultrasonic signals transmitted according to step (E), G. counting the electric pulses generated according to step (F) for a period proportionate to t2 - t1 to determine a term proportionate to (t2 - t1)/t0, said step (G) constituting a third measuring period and H. counting the electric pulses generated according to step (F) for a period proportionate to (t2 - t1)/t0 to determine the term (t2 - t1)/t02 representing the speed of the fluid in said conduit, said step (H) constituting a fourth measuring period.
 3. A method as defined in claim 2, wherein the counting steps (B) and (D) are performed by a first counter at a relatively high clock pulse frequency, the counting step (G) is performed by a second counter during an emptying of the contents t2 - t1 of the first counter at the rate of a relatively low clock pulse frequency and the counting step (H) is performed by a third counter during an emptying of the contents (t2 - t1)/t0 of the second counter at the rate of said relatively low clock pulse frequency. 